1. Field of the Invention
This invention relates to a method of resistance welding articles made of dissimilar metals, one of which is formed with a projection; more particularly it relates to resistance projection welding of steel with aluminum, steel with cast iron, copper with aluminum and steel with titanium, as well as a weld joint produced by the same method.
2. Description of the Prior Art
Techniques for electrical resistance welding of two metallic articles have been known for many years, including the use of a projection or projections on the surface of one of the articles to be welded (so-called projection welding). The use of such projections permits matching the heat capacities of the two articles, for example, as when welding a relatively thick member to a thin sheet.
Conventional projection welding methods involve either fusion welding or pressure (diffusion) welding. In fusion welding, the metallic materials of the member having the projection and the other member melt, and the final welded joint contains a nugget of such materials that have melted and then resolidified. The high temperature in the melt zone facilitates the formation of a solid solution of foreign materials and oxides that may be present on the metallic surfaces to be joined. Consequently, the fusion welding method does not normally require extensive precleaning of the contacting surfaces. On the other hand, since many dissimilar metals tend to form brittle intermetallic compounds when melted together, which will result in a weak joint, fushion welding is usually restricted to the bonding of articles made of the same metal.
The welding parameters used in conventional fusion welding typically include a moderate welding current applied over a relatively long time duration. For example, fusion welding of steel to steel may involve a welding current density of about 1 KA/mm.sup.2 applied over 20 to 30 cycles. (Throughout the Specification and claims, welding times will be expressed in terms of cycles of alternating current, a frequency of 60 Hz being understood.) There are no established standard values for the welding pressures to be used in conventional projection welding. However, conventional welding pressure in general is relatively low. For example, in the related art of spot welding, the RWMA, Class A standard recommends a welding pressure of 14 kg/mm.sup.2, for a welding current density of 0.24 KA/mm.sup.2 and a current duration of 40 cycles.
In diffusion welding, the two metal articles remain solid and are bonded by applying sufficient pressure at a temperature below their melting points to cause diffusion across the boundary between them. Because welding takes place below the melting temperatures of the metals, any foreign matter on the surfaces will form undissolved inclusions in the welded joint. Also, because diffusion welding takes a relatively long time, intermetallic compounds will tend to form even at the below melting point temperatures used for this method.
Consequently, successful diffusion welding usually requires preliminary chemical purification of the weld surfaces to remove foreign materials and oxides, and it is conducted either under vacuum or in an inert atmosphere to maintain surface purity over the long period required for bonding. Difffusion welding, therefore, finds application only within a limited field of industry and is not well suited to mass production methods.
Thus, resistance welding of dissimilar metals poses great difficulties. On the one hand, the foreign matter and oxides usually present on the surfaces of the metals will not readily go into a solid solution with either of the metals at the relatively low temperatures used for diffusion welding. On the other hand, the formation of brittle intermetallic compounds is accelerated at the higher temperatures used in fushion welding.
The prior art has long recognized that the intermetallic compounds and oxidized surface layers can be physically displaced away from the weld region by exerting high pressure while the metals adjacent to the junction are heated to the plastic state. Under the influence of such pressure exterted normal to the plane of the joint, the plastic metals at the boundary between the articles being welded will be squeezed laterally to the edges of the joint, carrying the extraneous materials with them. The resulting weld bond is thereby formed between unmelted sub-surface portions of the joined articles that have not been exposed to the atmosphere.
For example, U.S. Pat. No. 651,597, issued in 1900 to R. Eyre, discloses a method for butt-welding steel bars by bringing the two pieces in contact with each other and passing a heating current through them. When the central portion has reached the desired temperature, which preferably should not be more than a dull red heat, the joint is upset with sufficient pressure and through a sufficient distance to squeeze substantially all of the metal in the central heated zone to the periphery of the junction. Hence, the actual weld is formed by metal which has never been exposed to the atmosphere.
The Eyre patent emphasizes that there must be a sharp contrast in temperature between the central portion of the weld zone and the adjacent portions; so that the actual union is made between metal of each bar that has been maintained at a relatively low temperature. Eyre teaches the use of a stream of water to cool the regions of the workpieces away from the junction and thereby achieve the desired temperature gradient.
Although simple in concept, this procedure is difficult to control in practice. Relatively small variations in amplitude and duration of welding current and in welding pressure can affect substantially the plasticity of the metals at and near the junction. The problem is compounded when the pieces to be joined are made of dissimilar metals having different melting points, different electrical conductivities, and different specific heats.
One approach to controlling the heat conditions in a weld zone between two dissimilar metals is proposed in U.S. Pat. No. 3,435,183, issued May 19, 1965 to J. J. Vagi. The Vagi patent relates to a method for continuously welding a thin aluminum fin in a helix on a stainless steel tube. The method calls for passing a high frequency welding current between a first electrode contacting the aluminum fin and a second electrode contacting the pipe while the pipe is rotating. The aluminum fin is wound around the periphery as the pipe rotates. The first electrode is spaced a predetermined distance from the initial contact point between the aluminum fin and the pipe, and the second electrode is spaced circumferentially a predetermined distance from the same contact point on the pipe.
The power level of the current is just sufficient to render plastic only the surfaces of the tube and of the aluminum strip adjacent to the juncture between them. Upon application of welding pressure at that juncture, intermetallic compounds and oxidized surfaces formed on the plasticized portions of the tube and strip are displaced from the immediate area of the juncture to bring the plastic unoxidized surfaces of the aluminum and steel into engagement to form a bond.
According to the patent specification, uneven heating due to unavoidable variations in heating effect and parameters of the materials can be controlled by selecting an optimum peripheral velocity and by spacing the first and second electrodes from the weld juncture by a predetermined differential spacing. The method of the Vagi patent for controlling the temperature of the two materials is not suitable, however, for use where the articles to be welded must be clamped in the respective electrodes.
Another approach to controlling conditions in the weld zone, again when welding dissimilar metals, is provided in U.S. Pat. No. 3,089,021, issued May 7, 1963 to D. H. Hawes, et. al. In the method disclosed in the Hawes patent, a relatively low-current arc is formed between two members that are not in contact. This arc is maintained until adjacent first portion of the workpieces are softened and melted, and second portions are plasticized. Only then are the two members brought into contact under great pressure, thereby expelling the molten material from the weld zone as flashing; so as to produce an intimate bond between the heated plastic materials of the second portions.
Thus, the welding method of Hawes provides temperature control by means of the inherent current limiting characteristic of an electric arc. Because of this limitation, which produces current densities of about 12,000 amperes per square inch (or about 0.02 KA/mm.sup.2), the arcing time must extend for 10 to 60 seconds in order to produce the desired temperature conditions.
Still another method for controlling the heat in a resistance weld between dissimilar metals is presented in U.S. Pat. No. 910,434, issued on Jan. 19, 1909 to C. E. Thompson. In the Thompson process, metal parts having different electrical conductivities are resistance welded by first proportioning the contacting areas of the two parts inversely to their respective conductivities.
In a specific example, a brass head is welded to a steel bolt, the head having a welding lug of frusto-conical form. Since the conductivity of brass is nearly five times that of steel, the tip of the conical lug is formed with a cross-sectional area approximately one-fifth the area of the steel bolt. The two parts are pressed together under steady pressure as electric current passes through them.
According to the patent specification, the reduced contact area at the tip of the welding lug limits the flow of current so that the rise in temperature of the brass head can be controlled, and the continuous end of the bolt will become plastic before the head becomes so fluid as to run. The brass head closes in as the lug fuses, and by the time that the conical portion has been fused, the steel is brought to the required welding temperature.
The Thompson patent does not specify the use of high pressure to expel the fused material away from the weld juncture. Instead, the patent drawings indicate that a moderate pressure is used, since only a modest bulge is shown at the plane of the weld joint. The specification does mention, however, that filaments or fringes of the respective materials interlock at the weld joint, and the patentee takes this to indicate that the two materials are substantially equally plasticized in spite of their unlike conductivities. Since no values of welding current, welding time, or welding pressure are given, however, this observation cannot be verified.
The prior art of projection welding includes German Auslegeschrift No. 1,067,545, published on Oct. 22, 1959. This German patent discloses a fusion welding process, in which the first half wave of welding current is suppressed with respect to the succeeding half waves, in terms of a current-time-integral. This German patent discloses that a relatively large current density and welding pressure, for example, a current of 35.5 KA and a welding pressure of 320 kg are applied to a projection having a diameter of 3 mm and a height of 0.4 mm. Since the diameter of the projection is larger than the height thereof, it is difficult to produce a weld joint having a high strength from the dissimilar metals mentioned above, namely, steel with aluminum, steel with cast iron, copper with aluminum and steel with titanium.